I have read, and heard confirmed by my friend who's an orthodontist, that you should brush your children's teeth for them until they are five. I think by the later years it may be more monitoring, but if your daughter is anything like mine, her idea of brushing is putting on the toothpaste herself (or trying) and chewing on the brush. I brush mine with her. After she has her turn, I get another toothbrush (that hasn't been destroyed by her methods!), a character one that lights up for the recommended minute, and sit on the floor with her in front of a mirror, and as I brush I sing the 'Brusha-brusha' song. (I made it up; basically improv about brushing to the tune of Twinkle Twinkle Little Star). Most of the time she sits well for it, especially if I've let her brush first, but even when she fusses it's not too bad. Heck, when she yells it makes it easier to reach the teeth! It's just too important an issue to let this one go, in my opinion.
Good luck!